Transmission Line Impedance Calculator
Calculate the characteristic impedance of a coaxial cable from its physical dimensions and dielectric constant.
PTFE=2.1, PE=2.3, Air=1.0
Characteristic Impedance
57.6 Ω
Characteristic Impedance vs Outer Conductor Inner Diameter
Coaxial Cable Characteristic Impedance
The characteristic impedance of a coaxial transmission line depends on the ratio of conductor diameters and the dielectric material.
Formula
Z_0 = (138 / sqrt(Er)) x log10(D / d)
Where D is the outer conductor inner diameter, d is the inner conductor outer diameter, and Er is the relative dielectric constant.
Common Values
The velocity factor is 1/sqrt(Er), which determines signal propagation speed.
Example Calculation
Coaxial cable with D=7.0 mm, d=1.63 mm, polyethylene dielectric (Er=2.3).
- 01Diameter ratio: 7.0 / 1.63 = 4.294
- 02log10(4.294) = 0.633
- 03Z0 = 138 / sqrt(2.3) x 0.633 = 138 / 1.517 x 0.633 = 57.6 ohms
- 04Velocity factor: 1 / sqrt(2.3) = 0.659
Frequently Asked Questions
Learn More
Ohm's Law Guide
A complete guide to Ohm's Law. Learn the relationship between voltage, current, and resistance, with practical examples, the power triangle, and circuit analysis tips.